xml: Add element <title> to allow short description of domains

xml: Add element <title> to allow short description of domains

This patch adds a new element <title> to the domain XML. This attribute
can hold a short title defined by the user to ease the identification of
domains. The title may not contain newlines and should be reasonably short.

 *docs/formatdomain.html.in
 *docs/schemas/domaincommon.rng
        - add schema grammar for the new element and documentation
  *src/conf/domain_conf.c
  *src/conf/domain_conf.h
        - add field to hold the new attribute
        - add code to parse and create XML with the new attribute